They're not just radicals. They are dedicated to forming a worldwide Caliphate. This is NOT the Iraq War of GW Bush. He started a war on false pretenses to stop supposed WMD. Had we left Iraq alone we could've even pursued cooperation with Saddam in stopping terrorists as Saddam hated Al Qaeda about as much as we did.
We should send troops into Iraq and Syria and help fight against the extremists. We should get Assad and the non-ISIS rebels to agree to a temporary ceasefire just like the Chinese did when the Japanese invaded in WWII. Maybe in fighting together they can even work out an agreement for peace.
The USA has tangoed with Iraq before and it wasn't good at all ... Remeber the whole George Bush - weapons of mass destruction escapade ? If you don't it's this fruitless intervention when geroge Bush led America into Iraq and got over 500000 civilians killed ... Learnt your lesson ?